hw/core: properly terminate loading .hex on EOF record

hw/core: properly terminate loading .hex on EOF record

The https://makecode.microbit.org/#editor generates slightly weird
.hex files which work fine on a real microbit but causes QEMU to
choke. The reason is extraneous data after the EOF record which causes
the loader to attempt to write a bigger file than it should to the
"rom". According to the HEX file spec an EOF really should be the last
thing we process so lets do that.

Reported-by: Ursula Bennée <[email protected]>
Signed-off-by: Alex Bennée <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Message-Id: <20200403191150[email protected]>